The present invention relates generally to modems. More particularly, the invention relates to a modem that generates modulated waveforms and serial binary data based on state vectors that form a tetrahedron in four dimensional space.
Modulator/demodulators (modems) have been in use for years, and are essential components in most communication systems, including digital radios. The primary modulation task of phase shift keying (PSK) modems is to convert input serial binary data into an output waveform that is modulated to discrete phase angles. The resultant phase states (or symbols) are essentially a representation of the binary data in the phase domain. Similarly, these modems demodulate by converting an input phase modulated waveform into output serial binary data. The basis for converting between the binary data and the waveform symbols is often termed the modulation xe2x80x9cschemexe2x80x9d or xe2x80x9ctechniquexe2x80x9d and typically dictates the applicability is of a modem to a given application.
In transmitting digital radio signals, most satellite communication systems utilize constant envelope (or constant amplitude) modulating techniques. Constant envelope techniques are common because saturating high power amplifiers are typically used in these applications and it is desirable to minimize signal distortion. Characteristically, saturating high power amplifiers are more efficient than linear amplifiers, and amplitude based modulation schemes, such as quadrature amplitude modulation (QAM), have difficulty coping with the non-linearity of these amplifiers.
Furthermore, while many constant envelope modulation techniques have evolved such as bi-polar phase shift keying (BPSK), quadrature phase shift keying (QPSK), and variants thereof, methods for improving bit error rate (BER) for a given channel bandwidth remains a topic of intense study in the communications field.
While the above modulation techniques have become standard methods for communicating data, some factors reduce their desirability with respect to digital radio transmissions. For example, unencoded PSK signals have an inherent phase ambiguity that the receiver must resolve. This can be avoided by using a differentially encoded PSK waveform (i.e. mark or space encoded rather than level); unfortunately, this doubles the theoretical BER for a given signal to noise ratio.
Another problem relates to bit synchronization. Specifically, digital communication receivers must establish bit synchronization with the recovered waveform to properly detect data. This is done by tracking symbol transitions. For a non-return-to-zero (NRZ) encoding scheme, however, there may be times when the incoming signal does not have enough transitions to maintain tight bit synchronization. This can lead to a high BER or even temporary loss of bit synchronization. It is therefore desirable to provide a modulation scheme that ensures a phase transition during each symbol period to aid bit synchronization and detection. Bi-phase (a.k.a. Manchester) codes are sometimes used to aid bit synchronization in the demodulator. This is done by ensuring a phase transition during each bit period by encoding the clock onto the data. Bi-phase codes, however, decrease the throughput of a BPSK signal by xc2xd (i.e. the throughput of a bi-phase waveform is equal to xc2xc bits per second/Hz). Scrambling the signal is another approach to ensure enough symbol transitions for adequate bit synchronization, but this approach adds complexity and requires synchronization which is often undesirable or even unacceptable due to delays that occur while the scrambling code is being acquired. There are various encoding schemes that are used to improve the BER of standard BPSK and QPSK links (such as convolutional encoding/Niterbi decoding, or block encoding). However, these approaches typically add complexity and cost to the communication system, and decrease the transmission throughput. Although forward error correction (FEC) encoding can be used to resolve the PSK waveform ambiguity, it requires additional detector control software and synchronization time.
The throughput (bandwidth efficiency) of a modulation scheme is often a major consideration when designing for band limited channels. Unfortunately, techniques used to improve the BER or aid bit synchronization, such as those described above, typically lower the channel throughput. It is therefore desirable, if possible, to provide a modulation scheme that improves BER and aids bit synchronization without degrading throughput. Furthermore, it is also desirable that the modulation scheme produce a constant envelope waveform, which enables transmission through non-linear devices such as saturating amplifiers commonly found in satellites. Lastly, it is desirable to have a modulation scheme that is not phase ambiguous.
The above objectives are provided by a system and method in accordance with the present invention for phase modulating data and demodulating input waveforms. A tetrahedron modem includes a modulation system that converts data into phase modulated symbols based on a state vector set. The demodulation system converts the phase modulated waveform back into the original data based on coherent detection of these symbols. The vectors that describe the symbol states are selected to form a tetrahedron in four dimensional space, which ultimately improves the BER for a given SNR. In addition, throughout this process the modulated waveform maintains a constant envelope, which permits the use of non-linear devices such as saturating amplifiers without signal distortion.
In accordance with the present invention, a method for creating a plurality of dimensions is described. The approach taken here is to utilize the symbol clock along with the inphase (l) and quadrature (Q) signals from the carrier local oscillator to generate the four required dimensions. A state vector set is defined such that the modulating symbols form a tetrahedron within this four dimensional space. The modulation manifests itself as a constant amplitude signal with specific phase angles and transitions based on the state vector set.
The present invention also provides a method for demodulating a phase modulated waveform. The method includes the step of creating a plurality of dimensions similar to that found in the modulator. Demodulation vectors are defined to allow coherent detection of the tetrahedral waveform within the four dimensional space. The phase angles and transitions are converted into data based on this vector set.